A bug in Ecartis, the mailing list software at FreeLists, is to blame. Fortunately, there is an easy workaround: always ensure that you send the post approval message (the one that goes to your -repost address) in plain text and without any attachments. Although it isn't needed, many mail clients will create one automatically when forwarding the moderator message and this breaks the moderation process. Simply remove the attachment before sending the email.